Improving accuracy of electron density measurement in the presence of metallic implants using orthovoltage computed tomography

Abstract

The goal of this study was to evaluate the improvement in electron density measurement and metal artifact reduction using orthovoltage computed tomography (OVCT) imaging compared with conventional kilovoltage CT (KVCT). For this study, a bench-top system was constructed with adjustable x-ray tube voltage up to 320 kVp. A commercial tissue-characterization phantom loaded with inserts of various human tissue substitutes was imaged using 125 kVp (KVCT) and 320 kVp (OVCT) x rays. Stoichiometric calibration was performed for both KVCT and OVCT imaging using the Schneider method. The metal inserts-titanium rods and aluminum rods-were used to study the impact of metal artifacts on the electron-density measurements both inside and outside the metal inserts. It was found that the relationships between Hounsfield units and relative electron densities (to water) were more predictable for OVCT than KVCT. Unlike KVCT, the stoichiometric calibration for OVCT was insensitive to the use of tissue substitutes for direct electron density calibration. OVCT was found to significantly reduce metal streak artifacts. Errors in electron-density measurements within uniform tissue substitutes were reduced from 42% (maximum) and 18% (root-mean-square) in KVCT to 12% and 2% in OVCT, respectively. Improvements were also observed inside the metal implants. For the detectors optimized for KVCT, the imaging dose is almost doubled for OVCT for the image quality comparable to KVCT. OVCT may be a good option for high-precision radiotherapy treatment planning, especially for patients with metal implants and especially for charged particle therapy, such as proton therapy.

Does kV–MV dual-energy computed tomography have an advantage in determining proton stopping power ratios in patients?

“…In our previous study, the use of a 320-kVp beam substantially reduced the metal artifact caused by a Ti insert (Yang et al , 2008). The 1-MV beam should reduce the metal artifact even more, which will make it easier for physicians to draw contours in the artifact-affected area.…”

Use of a megavoltage electronic portal imaging device to identify prosthetic materials

“…Attempts have been made to overcome this problem using an extended CT number range [6], orthovoltage cone beam CT [7], and megavoltage cone beam CT [8,9]. However these techniques are either uncommon or experimental and are not available in many clinical departments [10].…”

Evaluation of CT calibration curves from stoichiometric and tissue substitute methods according to tissue characteristics

“…Frequently, X-ray computed tomography (CT) is used in radiotherapy treatment planning systems as a basic source of information about the exact anatomical locations of the inhomogeneities to correct tissue heterogeneities (Cozzi et al, 1998). Since CT scans are worked at diagnostic X-ray energies (up to 140 kVp), the CT numbers or Hounsfield units (HUs) should be converted into physical properties of human tissues (such as electron densities), before they become applicable for calculating doses at higher X-ray energies (Yang et al, 2008). The correlation between the CT numbers, and electron densities (r e ) of different tissues (CT calibration curve), is the prerequisite for accurate patient dose calculations in radio-therapy treatment planning systems (Yang et al, 2008;Tsukihara et al, 2015).…”

“…Since CT scans are worked at diagnostic X-ray energies (up to 140 kVp), the CT numbers or Hounsfield units (HUs) should be converted into physical properties of human tissues (such as electron densities), before they become applicable for calculating doses at higher X-ray energies (Yang et al, 2008). The correlation between the CT numbers, and electron densities (r e ) of different tissues (CT calibration curve), is the prerequisite for accurate patient dose calculations in radio-therapy treatment planning systems (Yang et al, 2008;Tsukihara et al, 2015). The common method of displaying this calibration curve is tissue substitute method, in which, a number of different tissue equivalent materials with known physical properties is selected and scanned by desired CT scanner.…”